The first two are self-published (and probably overpriced at 99 cents .. even perhaps at free, although I haven't looked at these two specifically). The last, though, is from HarperCollins, is over $12 in print and is probably $10 usually. It still may be overpriced, but has (we hope), at least been seen by an editor, had some revisions to make sure info isn't repeated to often and isn't just a thinly veiled sales pitch into a scam (not that it might not be a sales pitch).

The Actually Useful Excel is now $2.99. The title sounds like the author doesn't think that the way that gazillions of people use Excel now is actually useful. Hmmm . . . . Well, I'll have to wait until it's marked down again to find out.